The present invention relates to borehole measurements and more particularly to the generation of temperature-compensated magnetic fields suitable for performing downhole measurements of fluids using nuclear magnetic resonance (NMR) techniques.
Performing measurements on fluid samples is desirable in many oil industry applications. In the prior art such measurements are typically made by bringing samples to the surface using sealed containers, and sending the samples for laboratory measurements. A number of technical and practical limitations are associated with this approach.
The main concern usually is that the sample(s) taken to the surface may not be representative of the downhole geologic formation due to the fact that only limited sample material from a limited number of downhole locations can be extracted and taken to the surface. Thus, taking samples to the surface is impractical if it is desired to measure the fluid on a dense grid of sample points. Therefore, by necessity the measurements will only provide an incomplete picture of the downhole conditions. In addition, these samples frequently contain highly flammable hydrocarbon mixtures under pressure. Depressurizing the containers frequently leads to the loss of the gas content. Handling of such test samples can be hazardous and costly. It is therefore apparent that there is a need for direct downhole fluid testing that would overcome these and other problems associated with prior art solutions.
Various methods exist for performing downhole measurements of petrophysical parameters of a geologic formation. Nuclear magnetic resonance (NMR) logging is among the most important methods that have been developed for a rapid determination of such parameters, including formation porosity, composition of the formation fluid, the quantity of movable fluid, permeability and others.
Some of the main formation parameters measured using NMR techniques include the parameter T1 (known as the spin-lattice relaxation time), which corresponds to the rate at which equilibrium is established in bulk magnetization in the formation upon provision of a static magnetic field. Another related and frequently used NMR logging parameter is the spin-spin relaxation time constant T2 (also known as transverse relaxation time), which is an expression of the relaxation due to non-homogeneities in the local magnetic field over the sensing volume of the logging tool. Both relaxation times provide indirect information about the formation porosity, the composition and quantity of the formation fluid, and others.
Another measurement parameter used in NMR well logging is the formation diffusion, which generally, refers to the motion of atoms in a gaseous or liquid state due to their thermal energy. It is well known that a correct interpretation of the NMR measurement parameters T1, T2 and diffusivity may provide valuable information relating to the types of fluids involved, the structure of the formation and other well logging parameters of interest. The accuracy of the measurements, and thus the validity of the derived information, depends on a number of factors, including the ability of the measurement tool to provide consistent measurement results over a wide range of practical conditions, such as the operating temperature.
NMR measurements of geologic formations may be done using, for example, the centralized MRIL(copyright) tool made by NUMAR, a Halliburton company, and the side-wall CMR tool made by Schlumberger. The MRIL(copyright) tool is described, for example, in U.S. Pat. No. 4,710,713 to Taicher et al. and in various other publications including: xe2x80x9cSpin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination,xe2x80x9d by Miller, Paltiel, Millen, Granot and Bouton, SPE 20561, 65th Annual Technical Conference of the SPE, New Orleans, La., Sep. 23-26, 1990; xe2x80x9cImproved Log Quality With a Dual-Frequency Pulsed NMR Tool,xe2x80x9d by Chandler, Drack, Miller and Prammer, SPE 28365, 69th Annual Technical Conference of the SPE, New Orleans, La., Sep. 25-28, 1994). Details of the structure of the MRIL(copyright) tool and the measurement techniques it uses are also discussed in U.S. Pat. Nos. 4,717,876; 4,717,877; 4,717,878; 5,212,447; 5,280,243; 5,309,098; 5,412,320; 5,517,115; 5,557,200; 5,696,448; 5,936,405; 6,005,389; 6,023,164; 6,051,973; 6,107,796 and 6,111,408, all of which are commonly owned by the assignee of the present application. The Schlumberger CMR tool is described, for example, in U.S. Pat. Nos. 5,055,787 and 5,055,788 to Kleinberg et al. and further in xe2x80x9cNovel NMR Apparatus for Investigating an External Sample,xe2x80x9d by Kleinberg, Sezginer and Griffin, J. Magn. Reson. 97, 466-485, 1992. The content of the above patents and publications is hereby expressly incorporated by reference for background.
Wireline logging of boreholes performed using the NMR tools described above or other techniques known in the art provides valuable information concerning the petrophysical properties of the formation and, in particular, regarding the fluid composition of the formation. However, additional fluid parameter information can be critical for the interpretation of the wireline NMR measurements. For example, it is often desirable to distinguish between water, connate oil, drilling mud filtrates and gas based on the differences in T1, T2 and diffusivity. The true values for connate oil and the drilling mud filtrates under reservoir conditions are often unknown and must be approximated from laboratory measurements done under different conditions. Therefore, for increased accuracy, it is desirable to perform real-time downhole NMR determination of the T1, T2 and diffusivity parameters of borehole fluids to enhance the quality and reliability of the formation evaluation obtained using the standard measurements.
Direct downhole measurements of certain fluid properties are known in the art. Several commercially available tools may be used to this end. Examples include the RDT tool manufactured by Halliburton, the Reservoir Characterization Instrument (RCI) from Western Atlas, and the Modular Formation Dynamics Tester (MDT) made by Schlumberger. These tester tools have modular design that allows them to be reconfigured at the well site. Typically, these tools provide pressure-volume measurements, which may be used to differentiate liquids from gases, and which are also capable of providing temperature, resistivity and other mechanical or electrical measurements. However, none of these tools is presently capable of providing NMR measurements, such hydrogen density, self diffusivity, or relaxation times.
A tester capable of performing direct downhole NMR measurements that can be used to enhance the quality and reliability of formation evaluation and that can provide a modular NMR downhole tester as an add-on to existing testing equipment to minimize the cost of extra measurement is disclosed in U.S. Pat. No. 6,111,408, entitled xe2x80x9cNuclear Magnetic Resonance Sensing Apparatus and Techniques For Downhole Measurementsxe2x80x9d by the present inventors.
The ability of such a tester to perform rapid and accurate NMR measurements is critically dependent on the ability to produce an intense and uniform magnetic field in the test vessel containing the fluids of interest. A downhole tester, such as that disclosed in U.S. Pat. No. 6,111,408, is exposed to extreme changes of temperature and the magnetic field generated in the test vessel of such a downhole tester must show little drift with temperature and should not be influenced by external fields and materials.
Therefore, there is a need for an apparatus for the generation of a magnetic field in a test vessel of a downhole tester, which is able to generate a static magnetic field of high field strength, spatial uniformity and minimal drift over a wide temperature range suitable for practical applications, e.g., 0xc2x0 C. to 175xc2x0 C. In addition, there is a need for an apparatus, the magnetic field of which is largely confined to an interior volume of the test vessel, so that the influence of external fields and materials is minimized or eliminated.
Described herein is an apparatus for the generation of a spatially uniform and temperature compensated magnetic field, within a given volume for use with nuclear magnetic resonance (NMR) techniques. In particular, an apparatus for generating a magnetic field suitable for a modular NMR tester capable of making direct downhole NMR measurements of various parameters of fluid samples of geologic formations near the walls of a borehole is provided. The modular tester is preferably incorporated as an add-on part to a standard commercial downhole formation tester.
In operation, test fluids located proximate to the borehole are introduced into a test chamber of the tester. In a preferred embodiment, the tester comprises a vessel made of a non-conductive material and a surrounding permanent magnet that creates a uniform, static, temperature-compensated magnetic field within the test chamber. In a preferred embodiment, a radio frequency (RF) coil is embedded in the walls of the vessel and is used to induce an excitation field with a direction perpendicular to the static magnetic field. NMR signals from the excited nuclei in the fluids are detected to obtain data for directly estimating a number of fluid parameters, or to assist in the interpretation of wireline MRIL measurements.
More specifically, in a preferred embodiment, an apparatus is provided for generating a uniform, temperature-compensated magnetic field suitable for use in a downhole NMR tester such as that disclosed in U.S. Pat. No. 6,111,408 to the present inventors, which is hereby incorporated by reference for all purposes. According to one embodiment, the apparatus comprises: a first magnet having longitudinal axis, magnetized in a direction that is perpendicular to the longitudinal axis; and at least a second magnet also in the form of a tube that is co-axial with and is within the first magnet tube. The second magnet is also magnetized in a direction that is perpendicular to its longitudinal axis and at some angle, preferably 0xc2x0 or 180xc2x0, to the magnetization direction of the first magnet.
In one aspect of the invention, the first and second magnets are composed of materials having different magnetic temperature coefficients and therefore the combined magnetic field within the internal volume of the magnets will vary less with temperature than the field within a comparable magnet assembly having just one type of magnetic material.
In another aspect of the invention, the apparatus further comprises a tubular pressure barrel co-axial with and containing both the first and second magnets. In a preferred embodiment, the pressure barrel is made of metal with a high relative magnetic permeability, such as soft-iron or low-carbon steel, and thus provides a low resistance path for magnetic field lines outside the magnets. This return path for the magnetic flux increases the magnetic flux density within the interior volume of the magnets.
In addition, in a preferred embodiment of the apparatus, the cross-sectional area of the first magnet and the cross-sectional area of the second magnet are selected in a ratio proportional to the ratio of the inverse of their respective temperature coefficients to enhance the temperature compensation effect. Alternatively or in addition, the degrees of magnetization of the two magnets may be selected in accordance with their respective temperature coefficients.
In one embodiment, the two magnets have different magnetic temperature coefficients with the same sign, and the angle between the direction of magnetization of the first magnet and that of the second magnet is approximately 180xc2x0. In another embodiment, the two magnets have different magnetic temperature coefficients with opposite signs, and the angle between the direction of magnetization of the first magnet and that of the second magnet is approximately 0xc2x0.
In particular, in accordance with the present invention is provided an apparatus for generating a temperature-stabilized magnetic field for conducting nuclear magnetic resonance (NMR) measurements, the apparatus comprising: a first magnet that generates a first magnetic field B1 in a test region, wherein the first magnetic field has a component B1z in a direction z, which component increases with increasing temperature; and a second magnet that generates a second magnetic field B2 in the test region, wherein the second magnetic field has a component B2z in the direction z, which component decreases with increasing temperature.
In another aspect, an apparatus for generating a temperature-stabilized magnetic field is provided for conducting NMR measurements, the apparatus comprising: a first magnet with a first magnetization in a direction z; and a second magnet with a second magnetization in the direction xe2x88x92z; wherein the first and second magnets have magnetic temperature coefficients of the same sign. In an alternative embodiment, an apparatus for generating a temperature-stabilized magnetic field is provided for conducting NMR measurements, the apparatus comprising: a first magnet with a first magnetization in a direction z; and a second magnet with a second magnetization in the direction z; wherein the first and second magnets have magnetic temperature coefficients of opposite signs.